Criterion Validity of the 30‐Second Chair Stand Test for Lower‐Limb Muscle Strength in Adults With Severe Mental Disorders: The PsychiActive Project

BACKGROUND AND PURPOSE: Individuals with Severe Mental Disorders (SMDs) face a high risk of premature mortality and disability due to accelerated neuromuscular decline. Despite the critical need to monitor physical function, the lack of validated, time-efficient tools limits their clinical utility and hinders the integration of objective assessments into routine care. This gap limits the early identification of physical function impairments and the implementation of timely evidence-based interventions. This is the first study to evaluate the criterion validity of the 30-s Chair Stand Test (30s-CST) as a pragmatic indicator of lower-limb muscle strength in adults with SMDs, providing a feasible functional indicator to support routine clinical assessment. METHODS: Using a cross-sectional validation design (STROBE), this study was conducted with 91 adults (18-65 years) diagnosed with SMDs. Participants performed the 30s-CST and reference isometric Knee Extension Strength (KES) testing. Statistical analyses included Pearson's correlation (r), Intraclass Correlation Coefficients (ICC), and Bland-Altman plots to determine agreement and heteroscedasticity, with sub-analyses by sex, age, and body mass index. RESULTS: Analysis revealed significant moderate correlation and agreement (r = 0.56 [95% CI 0.40-0.69]; ICC = 0.54 [0.38-0.67]; both p < 0.001) for the total sample, with stronger results in the female subgroup. The 30s-CST demonstrated precision, with a prediction error lower than the within-subject variability of the reference method. Bland-Altman plots showed acceptable agreement, with narrow limits of agreement (-1.79-1.80), and analysis indicated homoscedasticity. DISCUSSION: The 30s-CST appears to be a pragmatic and time-efficient screening tool for identifying individuals with reduced lower-limb muscle strength in SMDs. Although limited by the lack of biomechanical specificity between dynamic and isometric methods, the test offers a balance between clinical feasibility and its ability to provide a functional estimate related to lower-limb muscle strength. Integrating this pragmatic screening tool into routine practice may support the assessment of functional performance in resource-constrained settings where more complex strength assessments are not readily available.